290 THE PROTOZOA 



whether it be the blood-sucking invertebrate, or possibly the flesh 

 or organs of another vertebrate infected with trypanosomes. 



Two methods of inoculative transmission of trypanosomes have 

 been distinguished ; in the one, known as the " direct " or " mechan- 

 ical" method, the parasites merely become contained in or adhere to 

 the proboscis of the blood-sucking intermediary when it sucks blood 

 from an infected animal ; and when it feeds a second time the try- 

 panosomes pass directly, and without having undergone any change 

 or development, into the second host ; in the other, known as the 

 " indirect " or " cyclical" method, the trypanosomes, when taken up 

 by the blood-sucking invertebrate, go through a developmental cycle 

 in it, at the end of which, but not before, they are " ripe " for inocu- 

 lation into a suitable vertebrate host. Comparing natural with 

 artificial processes of infection, in the direct method the blood- 

 sucking invertebrate may be said to play the role merely of an 

 injection-syringe, but in the indirect method it acts also as a culture- 

 medium, in which the parasite passes through various phases and 

 assumes forms quite different from those occurring in vertebrate 

 blood. Patton (393) has put forward the view r that transmission 

 is always by the direct method, and that the crithidial and other 

 forms found in the blood-sucking invertebrate are parasites of the 

 invertebrate alone, and have 110 connection with the trypanosomes 

 found in vertebrates ; but the number of cases in which it has 

 now been shown clearly that trypanosomes go through a definite 

 cycle in the invertebrate host disproves Patton's contention, and 

 renders it unnecessary to discuss it further. It is rather the 

 direct method that stands in need of further demonstration ; though 

 undeniably possible as a laboratory-experiment, it may be doubted 

 if it ever really occurs in Nature, and in any case it is probably to 

 be regarded as a purely accidental rather than a normal occurrence. 



It has been frequently asserted or assumed that trypanosomes 

 can pass from parent to offspring, by so-called " hereditary trans- 

 mission," in the invertebrate host, but convincing proof of this state- 

 ment is as yet lacking entirely. Attempts to prove hereditary trans- 

 mission by direct experiment have given, for the most part, negative 

 results, and the observation so frequently made, that leeches, tsetse- 

 flies, fleas, mosquitoes, etc., bred from the egg and not exposed 

 to infection, are entirely free from parasitic flagellates, affords cumu- 

 lative evidence against the existence of any such method of trans- 

 mission (c/. Kleiiie and Taute, 459). Brumpt (419), however, asserts 

 that T. inopinatum is transmitted hereditarily from parent to off- 

 spring of the leech Helobdella algira. According to Porter (554), 

 " Crithidia " melophagia of the sheep-ked is also transmitted from 

 parent to offspring in this insect ; and if, as is extremely probable, 

 the flagellate in question is the developmental phase of the trypano- 



